In an optical communications field, digital coherent communications systems in which a coherent detection scheme of dramatically improving frequency utilization efficiency is combined with digital signal processing are attracting attention. Compared to systems constructed based on direct detection, digital coherent communications systems are known to be capable of not only improving receiving sensitivity but also compensating for waveform distortion of a transmission signal caused by chromatic dispersion and/or polarization mode dispersion resulting from optical fiber transmission since a receiver receives a signal from a transmitter as a digital signal. For this reason, introduction of the digital coherent communications system as a next generation optical communications technique is being discussed.
Signal light received by a coherent receiver is multiplied by local oscillator light and converted into a baseband signal. In a laser oscillator that generates a carrier of signal light or local oscillator light, it is difficult to implement frequency stabilization by a phase-locked loop which is generally used in an oscillator for wireless communications, and a large frequency offset between an output frequency of a laser oscillator of a transmitter and an output frequency of a laser oscillator of a receiver occurs. In an actual optical communications system, a frequency offset reaches ±5 GHz. In a coherent communications system, since information is carried on the phase of a carrier, it is necessary for a receiver to estimate and compensate for a frequency offset.
Furthermore, in wireless communications, a frequency offset occurs due to errors of oscillation frequencies of reference oscillators used in a transmitter and a receiver and the Doppler shift associated with movement of a transmitter and a receiver. Even in this case, it is necessary for the receiver to estimate and compensate for a frequency offset.
A method using a phase increment algorithm utilizing phase shift information of a symbol during one symbol period, which is disclosed in Non-Patent Document 1, is known as a frequency offset estimating technique. However, in this method, an estimable frequency offset range is limited.
In contrast, Patent Document 1 discusses a method for estimating a frequency offset using the shape of a frequency spectrum of an orthogonal frequency division multiplexing (OFDM) signal. In this case, an estimable frequency offset range can be increased as compared to that in the phase increment algorithm.
FIG. 10 is a block diagram illustrating a configuration example of a conventional frequency offset estimating apparatus. Referring to FIG. 10, a frequency offset estimating apparatus 104 includes a serial-to-parallel converting circuit 5, a discrete Fourier transform (DFT) circuit 6, and a centroid calculating circuit 7. Furthermore, a first A/D converter 1, a second A/D converter 2, and a combining circuit 3 are connected to the frequency offset estimating apparatus 104 as peripheral circuits of the frequency offset estimating apparatus 104.
The first A/D converter 1 performs analog-to-digital conversion on an in-phase component (I signal) of a reception signal, the second A/D converter 2 performs analog-to-digital conversion on a quadrature-phase component (Q signal) of the reception signal, and then the combining circuit 3 converts the analog-to-digital converted signals into a signal of a complex number of I+jQ. An output signal of the combining circuit 3 is input to the frequency offset estimating apparatus 104.
FIGS. 11 and 12 are conceptual diagrams illustrating an operation of the conventional frequency offset estimating apparatus 104. FIGS. 11 and 12 each illustrate a frequency spectrum of the reception signal, which is an output signal of the discrete Fourier transform circuit 6. FIG. 11 illustrates a frequency spectrum when there is no frequency offset, and FIG. 12 illustrates a frequency spectrum when there is a frequency offset. Here, it is assumed that (1, 2, . . . , NS) are frequency numbers of a discrete Fourier transform, Pi is signal power of the frequency component of a frequency number i, and C is the frequency number corresponding to a reference frequency, and, for example, C is selected as NS/2. At this time, a signal δW/Wt proportional to the frequency offset is obtained through the following Formula (1).
                              [                      Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                            δ            ⁢                                                  ⁢            W                                W            t                          =                                            -                                                ∑                                      i                    =                    1                                    C                                ⁢                                                                  ⁢                                  P                  i                                                      +                                          ∑                                  i                  =                  C                                                  N                  s                                            ⁢                                                          ⁢                              P                i                                                                        ∑                              i                =                1                                            N                s                                      ⁢                                                  ⁢                          P              i                                                          (        1        )            